Typical steps of coacervation processes generally involve (a) emulsification of a generally hydrophobic material in a solution comprising hydrocolloids, (b) coacervation (phase separation) implying the formation of a coacervate phase (c) wall formation by aggregation of the hydrocolloid around droplets of the emulsified hydrophobic material, and, (d) wall-hardening, which is generally achieved by cross-linking the hydrocolloid forming the wall thus rendering the process irreversible and making the resulting microcapsules insoluble in water, resistant to mechanical stress and to heat exposure.
The step of wall formation is generally driven by the surface tension difference between the coacervate phase, the water and the hydrophobic material. In most industrial coacervation processes, one of the hydrocolloids used in coacervation processes are selected from gellable proteins. These are easier to use and less prone to aggregation after the formation of the wall when the temperature is below the gelling temperature, if compared to non-gellable hydrocolloids. Gellification, in turn, is generally brought about by lowering the temperature of the reaction mixture below the gelling point of the gellable hydrocolloid. This well-recognized principle is illustrated in U.S. Pat. No. 2,800,457, where the process of a complex coacervation is disclosed in detail. In column 1 it is written: The mixture may thus be made by forming an aqueous sol of one colloid, emulsifying the selected oil therein, and mixing the emulsion with an aqueous sol of another colloid, or the two sols may be made and mixed and the oil emulsified therein. [ . . . ] The process steps, down to the gelation step, are carried out with the ingredients at a temperature above the gel point of the colloid materials used, and gelation is brought about by cooling.
Similarly, GB 920,868 and WO 2004/022220 A1 both disclose forming the emulsions, which include the hydrophobic material to be encapsulated, at a temperature above the gel point.
Today, many industrial coacervation processes are still conducted according to this principle. In view of this, it is an objective of the present invention to establish different ways of manufacturing microcapsules by coacervation. In particular, it is an objective of the invention to reduce the duration of the heating step and of shortening the overall time of the process.
A further objective of the present invention relates to the wall-hardening step. For some years already, efforts have been made to replace glutaraldehyde and formaldehyde, due to their toxicity, as hardening agents by enzymatic treatment with transglutaminase for cross-linking the hydrocolloid. Transglutaminase is an enzyme having its temperature optimum in the range of 45-55° C. and its pH optimum at about 6-7.
Accordingly, U.S. Pat. No. 6,475,542 B1 and U.S. Pat. No. 6,592,916 B2 disclose simple coacervation processes, in which the cross-linking step is started at 30° C. but then conducted at an elevated temperature of 40° C., being closer to the temperature optimum of the enzyme. In these references it is mentioned that the enzyme reaction is usually carried out at 10 to 60° C.
In EP 0,856,355 A2 a complex coacervation process with cross-linking by transglutaminase is disclosed. According to this teaching, temperature during cross-linking in complex coacervation may be adjusted to 20° C. to 27° C. or to 5 to 10° C., but preferably the latter. At this low temperature, pH is preferably adjusted to the optimum pH, that is, 7. Likewise, in U.S. Pat. No. 6,969,530 B1, cross-linking is carried out at very low temperatures, preferably about 5° C.
In view of the prior art regarding cross-linking by transglutaminase it is an objective to provide a coacervation process in which transglutaminase is used at conditions that are different from those so far disclosed. In particular, it is an objective to provide an industrially viable method of micro-encapsulation by complex coacervation at temperatures not requiring prolonged cooling and/or heating to extreme temperatures. It is an objective to conduct cross-linking at about ambient or slightly above-ambient temperatures, but still below the enzyme optimum of 50° C., in order to avoid energy expenditure by excessive heating. Especially with the hardening step by transglutaminase, which generally takes from 5 to 24 hours and thus is the longest step of the entire manufacturing process, there is a vital interest in avoiding the maintenance of elevated (30° C. or above) or below-ambient temperatures (10° C. or lower). In other words, it is an objective of the present invention to provide an overall safer and more economic process for manufacturing microcapsules by coacervation.
It is an objective of the present invention to provide a process that is useful to encapsulate a large variety of different materials, including highly volatile and/or heat sensitive compounds. Flavours and fragrances frequently fall in this category. It is a particular objective to reduce loss of volatile components of the material to be encapsulated during the encapsulation process. For this reason it is an objective to provide a method of microencapsulation at comparatively low temperatures, thus avoiding loss of volatiles by evaporation. Furthermore, bioactive principles such as flavours, fragrances, drugs, for example, encompass heat sensitive compounds. For avoiding degradation of such compounds, low temperature encapsulation processes would provide an additional advantage.
Furthermore, the present invention has the objective of providing micro-capsules fulfilling worldwide religious and/or nutritional requirements. In particular, the present invention has the objective of using hydrocolloids and in particular gelatine that is kosher and/or halal.
In this respect, WO 96/20612 features the use of warm water fish gelatine in coacervation processes. Accordingly, it is taught that microencapsulation by complex coacervation has to be conducted at elevated temperatures, notably at temperatures of about 33-35° C. While the step of “microencapsulating” in this reference probably refers to the step of wall formation, it again becomes an objective of the present invention to manufacture micro-capsules at lower temperatures and hence, in a more economical way. It is in general an objective of the present invention to use warm water fish gelatine in a coacervation process, because it has been shown to provide capsule-walls having good heat-resistance and physical stability against shear forces. Furthermore, fish gelatine, while being kosher, is susceptible of obtaining halal status.
Given that the gelation temperature of warm water fish gelatine is generally above 27° C., it seems, according to current knowledge, virtually impossible to perform a complex coacervation process below this temperature. It is thus an objective of the present invention to use warm water fish gelatine in coacervation processes, and in particular complex coacervation processes and, at the same time, perform the step of micro-encapsulation (wall formation) below the temperatures indicated in the prior art.